1. Field of the Invention
The present invention is generally related to extremely short wavelength photolithography systems and, more particularly, to diffusely reflecting electromagnetic radiation in extreme ultraviolet (EUV) lithography systems.
2. Related Art
Lithography is a process used to create features on the surface of substrates. Lithography is a well known process in the art of manufacturing computer chips. A frequently used substrate for computer chips is a semiconductor material such as silicon or gallium arsenide. During lithography, a semiconductor wafer situated on a stage within a lithography tool is exposed to an image projected onto the surface of the wafer by an exposure system. The exposure system typically includes a reticle (also called a mask) for projecting an image of circuit features onto the wafer.
The reticle is generally located between the semiconductor wafer and a light source. The reticle is usually situated on a reticle stage within the lithography tool and is typically used as a photo mask for printing a circuit on a semiconductor chip. A light source shines through the mask and then through a series of optical lenses that reduce the image. This small image is then projected onto the semiconductor wafer. The process is similar to that used in a camera, which bends light to form an image on the photographic film.
The light plays an integral role in the lithographic process. For example, in the manufacture of microprocessors, one key to creating more powerful microprocessors is to reduce the wavelength of light used in the photolithographic process. A shorter wavelength allows the manufacture of smaller devices. Smaller devices, in turn, allow more transistors and other circuit elements to be etched onto a single silicon wafer, which yields more powerful, faster devices.
However, constantly shrinking wavelengths have yielded a number of challenges to chip manufacturers. For instance, the shorter wavelength of light, the more the light is absorbed in the glass optics intended to focus the light. As a result of this phenomenon, some light may fail to reach the silicon wafer, resulting in a degraded circuit pattern being created on the silicon wafer. As the wavelengths approach the extreme ultraviolet region of about 11-14 nanometers, glass material becomes more absorptive. For photolithography in this region—called Extreme Ultraviolet Lithography (EUVL)—glass lenses are replaced by mirrors, and the optical system is reflective, rather than refractive.
The problem of measuring the quality of the EUV illumination beam is a persistent one in EUVL applications. The use of shearing interferometry is one traditional method of optical system analysis. Use of shearing interferometry in refractive optical systems is well known. For a reflective optical system, such as is used in EUVL, various problems can arise. For example, in certain applications such as wavefront diagnostics, shearing interferometry requires a diffuse light source in the EUV range. Traditional refractive light diffusers do not work at such short wavelengths. It would be beneficial, then, to construct a reflective electromagnetic radiation diffuser that is operative at the extremely short wavelengths of EUVL systems.